Entrainment of a thermal with or without buoyancy reversal impinging on a stratified interface

Abstract

Laboratory experiments of a thermal with and without buoyancy reversal impinging on a stratified interface are presented. A thermal is created by releasing a small volume of buoyant fluid into a stratified environment composed of two layers of different densities. A thin interface separates the lower layer from the lighter upper layer. The entrainment of upper layer fluid into the thermal is investigated using a passive dye flow visualization technique. For the case of thermals without buoyancy reversal, the entrainment rate is found to obey a Ri$\sp{{-}3/2}$ power law, similar to the results obtained for stirring grid turbulence (Turner 1973) and a vertical plume impinging on a stratified interface (Kumagai 1984). This result was predicted by the stratified entrainment model of Cotel and Breidenthal (1997), using the concept of vortex persistence. The effect of simulated evaporative cooling on the entrainment of a thermal impinging on a stratified interface is also investigated experimentally. Evaporative cooling in atmospheric clouds is simulated by buoyancy reversal in the laboratory, where the mixed fluid is denser than either parent parcel. This is realized in the laboratory by releasing a mixture of ethyl alcohol and ethylene glycol in an aqueous solution. It rises first through a relatively dense lower layer fluid and then impinges on a thin stratified interface, above which is a layer of relatively light fluid. The entrainment of upper layer fluid across the interface is measured optically. The entrainment rate for values of the buoyancy reversal parameter, D$\sp{\*},$ between 0 and 0.5 was found to obey a Ri$\sp{{-}3/2}$ power law. The entrainment rate is independent of D$\sp{\*}$ between 0 and 0.5 for a range of Richardson numbers from 3 to 25. This is consistent with the behavior of the buoyancy-reversing thermal in an unstratified environment observed by Johari (1992).